500 STRUCTURE OF VERTEBRATA. 
organ, and myxcedema with its degeneration or absence. As injection 
of extract of sheep’s thyroid, or even eating this organ, alleviates myx- 
cedema, it is concluded that the thyroid must have some specific effect 
on the large quantity of blood which flows through it. It is probably 
safe to say that the thyroid aids in keeping the blood at a certain 
standard of health, through some specific secretion. 
The ¢hymus arises as a dorsal endodermic thickening where the 
outgrowths which form the gill-clefts meet the ectoderm. It may 
‘be associated with a variable number of clefts—seven in the shark 
Heptanchus, five in the skate, four in Teleosteans, three in the lizard, 
one in the chick, and one (the third) in Mammals. In the young 
lamprey there are said to be no fewer than twenty-eight thymus rudi- 
ments. In Mammals it often seems to degenerate after youth. In the 
rabbit it has its maximum weight in the fourth month, and thereafter 
begins to be rapidly reduced. As it has from its first origin a distinct 
lymphoid nature, and apparently forms leucocytes, it has been inter- 
preted (Beard) as a structure adapted for the phagocytic protection of 
the gills from bacteria, parasites, and the effects of injury. If this be 
so, we can understand its diminishing importance in Sauropsida and 
Mammalia, where its place may be to some extent taken by the palatal 
and pharyngeal tonsilsy which are believed by some (Stohr, Killian, 
Gulland) to have a similar phagocytic function. 
The pharynx leads into the gullet or cesophagus, which is 
a conducting tube, and this into the digestive stomach, 
which is followed by the diges- 
tive, absorptive, conducting 
intestine, ending in the rectum 
and anus. 
From the cesophagus the air- 
or swim- bladder of most Fishes, 
and the lungs of higher Verte- 
brates, grow out. The air- 
bladder usually lies dorsally and 
is almost always single; the 
lungs lie ventrally and are 
double, though connected with 
the gullet by a single tube. 
The beginning of the intes- 
tine gives origin to the liver, 
saa which regulates the composition 
ss er ee - "t. of the blood and secretes bile, 
chick. —After Goette. and to the pancreas, which 
a itera coe is shaded ; the endo- secretes oe juices. The 
erm dark. ancrea: i 
ég., One of the lungs; S¢., stomach ; P s has often a multiple 
Z., liver ; 4., pancreas, rudiment. 
